Visual experience influences the formation of nerve connections in the development of binocular input to the tectum of the frog, Xenopus laevis. Abnormal positioning of the eye in its socket leads to abnormal experience, which in turn alters the location of terminals from the nucleus isthmi to the contralateral tectum. However,there is some evidence that normal anatomical connections may persist as well but be suppressed physiologically. Anterograde labeling of isthmo-tectal arbors will be used to determine whether such a dual projection does exist and to test the hypothesis that the final pattern of isthmo-tectal topography is brought about by selective retention of physiologically appropriate parts of an initially widespread axonal arbor. The alternative hypothesis that selective cell death underlies isthmo-tectal topography also is being tested. The roles of the uncrossed isthmo-tectal projection to the tectum and of the noradrenergic innervation of tectum and NI in development of the crossed isthmo-tectal map also are being studied.